Intrusion alarm systems are known for detecting the presence of an intruder in a zone under protection. In such systems, there is typically provided a central control unit and a plurality of sensors, each located at an area to be protected, the sensors being interconnected by one or more multi-wired cables to the control unit. Typically, the sensors each include some means for detecting a predetermined condition, such as intrusion, waterflow, temperature, etc. The central control unit includes signal processing circuitry for detection of alarm signals indicating intruder presence.
While various systems have been suggested to detect intruder presence, and while various systems have been devised to prevent tampering with the system, such as sensor interlock switches to protect against dissembling the sensor, it is often times possible to defeat the intrusion alarm system by merely bridging the cable fore and aft of a particular sensor and then cutting the wires to the sensor between the bridge points so as to cut out the particular sensor. This effectively removes the particular sensor from the intrusion alarm cable supervision circuit and its absence is not readily ascertainable at the central control unit.
The defeating of the alarm system in this manner is possible when the sensors are connected in parallel along the interconnect cable.
For purposes of this application bridging refers to providing a conductive path or wire from a point just prior to a particular sensor to a point just aft of the sensor. Once this bridge has been established, it is then possible to cut out and remove the particular sensor from the system.